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Re: increasing the default WAL segment size

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Josh Berkus <josh@agliodbs.com>, Magnus Hagander <magnus@hagander.net>, Simon Riggs <simon@2ndquadrant.com>, "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2016-08-25T19:07:11Z
Lists: pgsql-hackers 
On Thu, Aug 25, 2016 at 2:49 PM, Alvaro Herrera
<alvherre@2ndquadrant.com> wrote:
> Robert Haas wrote:
>> On Thu, Aug 25, 2016 at 1:43 PM, Josh Berkus <josh@agliodbs.com> wrote:
>
>> > The one thing I'd be worried about with the increase in size is folks
>> > using PostgreSQL for very small databases.  If your database is only
>> > 30MB or so in size, the increase in size of the WAL will be pretty
>> > significant (+144MB for the base 3 WAL segments).  I'm not sure this is
>> > a real problem which users will notice (in today's scales, 144MB ain't
>> > much), but if it turns out to be, it would be nice to have a way to
>> > switch it back *just for them* without recompiling.
>>
>> I think you may be forgetting that "the base 3 WAL segments" is no
>> longer the default configuration.  checkpoint_segments=3 is history;
>> we now have max_wal_size=1GB, which is a maximum of 64 WAL segments,
>> not 3.
>
> I think the relevant one for that case is the minimum, though:
>
> #min_wal_size = 80MB
>
> which corresponds to 5 segments.  I suppose the default value for this
> minimum would change to some multiple of 64MB.

Yeah, Andres made the same point, although it looks like he
erroneously stated that the minimum was 48MB whereas you have it as
80MB, which seems to be the actual value.  I assume we would have to
raise that to either 128MB or 192MB, which does feel like a bit of a
hefty increase.  It doesn't matter if you're going to make extensive
use of the cluster, but if somebody's spinning up hundreds of clusters
each of which has very little activity it might not be an altogether
welcome change.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Make WAL segment size configurable at initdb time.

  2. Perform only one ReadControlFile() during startup.

  3. Introduce BYTES unit for GUCs.

  4. Remove useless duplicate inclusions of system header files.

  5. Refactor other replication commands to use DestRemoteSimple.

  6. Add a SHOW command to the replication command language.

  7. Add a new DestReceiver for printing tuples without catalog access.

  8. Support fls().

  9. Extend yesterday's patch making BLCKSZ and RELSEG_SIZE configurable to also

  10. Commit the reasonably uncontroversial parts of J.R. Nield's PITR patch, to

  11. XLOG (also known as WAL -:)) Bootstrap/Startup/Shutdown.

  12. Transaction log manager core code.